The invention relates to methods and apparatus for controlling pressure within a wellbore. In particular, embodiments of the invention comprise methods and apparatus for operating e ram-type blowout preventers.
Blowout preventers are used in hydrocarbon drilling and production operations as a safety device that closes, isolates, and seals the wellbore. Blowout preventers are essentially large valves that are connected to the wellhead and comprise closure members capable of sealing and closing the well in order to prevent the release of high-pressure gas or liquids from the well. One type of blowout preventer used extensively in both low and high-pressure applications is a ram-type blowout preventer. A ram-type blowout preventer uses two opposed closure members, or rams, disposed within a specially designed housing, or body. The blowout preventer body has bore that is aligned with the wellbore. Opposed cavities intersect the bore and support the rams as they move into and out of the bore. A bonnet is connected to the body on the outer end of each cavity and supports an operator system that provides the force required to move the rams into and out of the bore.
The rams are equipped with sealing members that engage to prohibit flow through the bore when the rams are closed. The rams may be pipe rains, which are configured to close and seal an annulus around a pipe that is disposed within the bore, or may be blind rams or shearing blind rams, which are configured to close and seal the entire bore. A particular drilling application may require a variety of pipe rams and blind rams. Therefore, in many applications multiple blowout preventers are assembled into blowout preventer stacks that comprise a plurality of ram-type blowout preventers, each equipped with a specific type of ram.
Ram-type blowout preventers are often configured to be operated using pressurized hydraulic fluid to control the position of the closure members relative to the bore. Although most blowout preventers are coupled to a fluid pump or some other active source of pressurized hydraulic fluid, many applications require a certain volume of pressurized hydraulic fluid to be stored and immediately available to operate the blowout preventer in the case of emergency. For example, many subsea operating specifications require a blowout preventer stack to be able to cycle (i.e., move a closure member between the extended and retracted position) several times using only pressurized fluid stored on the stack assembly. In high-pressure, large blowout preventer stack assemblies, several hundred gallons of pressurized fluid may have to be stored on the stack, creating both size and weight issues with the system.
Because many subsea drilling applications require the use of large diameter, high pressure blowout preventers, the height, weight, and hydraulic fluid requirements of these blowout preventers is an important criteria in the design of the blowout preventers and of the drilling rigs that operate them. Thus, the embodiments of the present invention are directed to ram-type blowout preventers that that seek to overcome these and other limitations of the prior art.